Conventional holding furnaces for accommodating melts of metals of a relatively low melting point, such as aluminum, generally have a lining of multilayer structure formed inside a shell case and comprising an outer refractory layer, an intermediate heat-insulating layer and an inner vessel layer for holding the molten metal in contact therewith. Stated more specifically, the lining is formed usually by lining the shell case with heat-insulating bricks, castable refractory or the like to provide the refractory layer, covering the refractory layer with a highly heat-insulating material such as heat-insulating bricks or heat-insulating fibers to form the heat-insulating layer, and further forming the vessel layer from ceramic board, castable refractory or the like (see, for example, JP-A No. 18248/1979 and Unexamined Japanese Utility Model Publication N. 30998/1985).
However, the molten metal holding furnace wherein heat-insulating bricks or heat-insulating fibers are used for the heat-insulating layer has the problem of failing to prevent the molten metal reaching the heat-insulating layer from penetrating into this layer, being insufficient in the heat-insulating properties of the heat-insulating material to necessitate an increased thickness for the heat-insulating layer and increase the size of the furnace, and requiring skill for the lining work.
Silica boards, castable refractories and the like which are in wide use for forming the vessel layer (lining wall) for containing the molten metal further have the problem of being liable to react with molten aluminum or with fluxes for treating molten metals. The reaction product is likely to cause formation of aluminum oxide or to produce hard spots by becoming incorporated into the resulting aluminum casting or to locally remove the surface of the inner furnace wall when dislodged from the wall. Stated in greater detail, the hard spot is the reaction product of molten aluminum and/or slag with the ceramic lining wall, consists mainly of molten metal oxide, nonmetallic inclusions and primary crystal metal and itself appears black because silica in the ceramic material is reduced to black silicon by aluminum. The hard spot has a very high hardness (about 2500 in Hv) and a high melting point (about 2000.degree. C.) and is therefore extremely difficult to remove. Accordingly, dislodging the hard spot from the lining wall frequently breaks the lining wall. The hard spot incorporated into the cast aluminum product renders the product faulty, so that it is very important to diminish the hard spots in affording the product in an improved yield.
Further in the case where silica board of high porosity is used for the lining wall, the flux for use in treating molten metal penetrates into the board, permitting the sodium component thereof to react with a component of the board and effecting the vitrification reaction of the board. When vitrified, the silica board exhibits impaired corrosion resistance to the molten metal, permitting seepage of the molten metal and cracking of the lining wall. The vitrified layer is higher than the base material in thermal conductivity, allowing a rise in the surface temperature of the shell case providing the outer periphery of the holding furnace and causing an increased amount of heat dissipation to result in an increased amount of power consumption for heating. Silica board is commercially available in the form of panels, which are joined to one another for use in lining. Accordingly, the board also has the drawback of necessitating skill for the lining work and permitting leakage of molten metal through the joint owing to the thermal stress repeatedly acting during use.
On the other hand, lining materials require a long period of time for drying when they are monolithic refractories containing a large amount of water (about 30 to 60 wt. %), such as castable refractories and calcium silicate slurries, because the lining material is positioned inside the outer peripheral shell case, has its outer periphery hermetically covered and therefore needs drying by heating from one side, i.e. from inside. If dried within a short period of time by rapid heating, the lining wall develops cracks and becomes damaged owing to the shrinkage of the material.
To solve the foregoing problems, proposals have been made which include a lining structure wherein a dry ramming material (up to 1% in water content) is used without addition of water to form a lining wall for use in molten metal vessels (Unexamined Japanese Utility Model Publication No. 1553/1990), and simplified lining techniques for forming a lining wall from an integral bath which is obtained first by a separate process, i.e., by shaping an integral structure from a monolithic refractory, followed by drying and firing (see JP-A No. 224289/1983, Unexamined Japanese Utility Model Publication No. 120526/1986, and JP-A No. 69744/1995).
However, the use of the ramming material involves a heavy burden of labor because the ramming material, which is almost free from water, encounters difficulty in ramming work which is essential to the formation of a homogeneous and compact lining. When the integral bath is used, there arises the problem that if the bath develops a crack, molten metal leaks out through the crack, in addition to the problem of penetration of the molten metal.
To inhibit hard spots, it has also been proposed to use a graphite-containing bath or to use a graphite crucible as an inner vessel (see JP-A No. 33486/1985 and Unexamined Japanese Utility Model Publication No. 67894/1991).
The graphite-containing bath is excellent in corrosion resistance and less likely to react with slag or flux and to produce hard spots, and therefore has the advantage of being usable for satisfactory casting. However, when developing a crack and permitting leakage of molten metal to the outside, the graphite-containing bath has the risk of causing the molten metal to melt the heat-insulating material within a short period of time and to leak out. If it is attempted to increase the graphite and silicon carbide contents to give improved resistance to thermal impact and to corrosion, higher thermal conductivity will result, entailing the problem of a reduction in the temperature of the molten metal due to the dissipation of heat. Thus, there are limitations on the improvement of characteristics by increasing the contents.
Further since the molten metal holding furnace is usually in the form of a box having a thin wall and a large opening, the lining material of furnace requires high costs in designing and making the mold, and extreme difficulties are encountered in preparing the graphite crucible by press forming. There are a wide variety of holding furnaces because they are not standardized in capacity, dimensions, etc. This leads to the problem that it is also economically difficult to prepare molds in conformity with the different kinds of furnaces.